1. Field of the Invention
The present invention relates to an electronic still camera in which the exposure time for a solid-state image sensor is controlled by a mechanical shutter.
2. Related Background Art
In a conventional electronic still camera in which the exposure time for a solid state image sensor is controlled by a mechanical shutter, the control operation is conducted according to a timing chart shown in FIG. 3, which shows an example of an interline transfer CCD (IT-CCD) as the solid-state image sensor.
In FIG. 3 there are shown a release signal 31, a vertical synchronization signal (Vd) 32, a signal (Vp) 33 delayed by about 700 .mu.s from the vertical synchronization signal Vd, a shutter operation signal 34 of which upshift and downshift respectively trigger the leading and trailing shutter curtains, a transfer gate (TG) pulse 35 for an odd field, and a transfer gate (TG) pulse 36 for an even field. The TG pulse 35 or 36 is a charge readout signal for each field, for transferring the charges of the light-receiving unit of the IT-CCD to the vertical transfer unit, has a pulse duration shorter than that of the signal Vp and is positioned within the duration of the signal Vp. There are further shown a recording period 37 for recording a frame image in the recording unit of the IT-CCD by field signal reading, and a mask control period 38 in which the charge transfer from the light-receiving unit to the vertical transfer unit by the TG pulses 35, 36 is prohibited.
Thus, in the conventional electronic still camera, in response to the entry of the release signal 31 at a time t31, the Vp signal 33 is given after a predetermined time (required for preparations for photographing such as mirror lifting) to start the running of the leading shutter curtain to initiate the exposure of the light-receiving unit of the IT-CCD. At t34, after the lapse of an arbitrary shutter time, the running of the trailing shutter curtain is started to complete the exposure operation. Subsequent to the completion of running of the trailing shutter curtain, when the mask control period ends at a time t35 in synchronization with the downshift of a pulse Vd, the signal readout is started in synchronization with the downshift, at a time t36, of an immediately following Vd signal 32. More specifically the signal charge transfer of an even field is started by a TG pulse 36 at the time t36, then the signal charge transfer of an odd field is started by a TG pulse 35 at a time t37, and a frame recording is completed at a time t38.
However, in such conventional drive control of the electronic still camera, if the shutter time is selected as shown in FIG. 3 for photographing an object with a high sensitivity, the time from the start of accumulation to the start of signal readout is a time T1 from t32 to t37 for an odd field but a time T2 from t33 to t36 for an even field, which are mutually different by two vertical periods (2 V=ca. 22 ms).
Such difference in the time from the start of charge accumulation to the start of charge readout between an odd field and an even field gives rise to a difference, between the odd field and the even field, in the amount of accumulated charge in the CCD due to the influence of dark current, eventually resulting in a flicker in case of interlaced reproduction of the recorded frame image on a television set, or in a density difference between neighboring lines, called line crawl, when the recorded frame image is transmitted and printed.